This application pertains to the art of fluid reservoirs and, more particularly, to the art of plastic tanks. The invention is particularly applicable for septic tank use and will be described with particular reference thereto, although it will be appreciated that the invention has broader applications such as subterranean storage of petroleum fuels, powdered chemicals, grain, or other fluid and non-fluid products.
Subterranean tanks for septic use have heretofore been constructed primarily of concrete although concrete block, fiberglass and steel tanks also have been used. For petroleum and fluid storage, fiberglass and steel tanks are used primarily. Such tanks are generally prefabricated. However, such tanks are, in some instances, fabricated on the installation site.
Septic tanks are commonly rectangular prisms of precast concrete. Near the top on one vertical end wall is an inlet for raw storage; near the top, on the opposite end wall is an outlet which permits treated fluids to pass to the septic field. Generally, there is an access opening in the top for removing accumulated solid materials which have settled to the tank bottom. To install the precast tanks, an excavation is made of sufficient size and depth that the outlet of the tank will be generally in the plane of the septic field. A hoist, often located on a transporting vehicle, is used to lower the tank into an excavation.
One of the principal problems with installing the prior art tanks in this manner, especially prefabricated concrete septic tanks, is the large weight. Installation is limited to firm, generally level ground able to support the heavy equipment necessary to lift and lower such tanks.
Another problem with the prior tanks has been flotation. In situations in which the ground water becomes high, the tanks tend to become bouyant. This bouyancy may cause the tank to shift vertically or even pop out of the ground.
Prior attempts to produce plastic subterranean tanks have failed to achieve successful products. Plastic tanks tend to be more expensive than their concrete, fiberglass, or steel counterparts unless the tanks are made very light weight. However, the prior art light weight tanks failed to achieve the strength required to prevent the tank from collapsing under subterranean soil loads.